This invention relates generally to tension mask cathode ray tubes (tubes), and particularly to support frames therein for maintaining the tension mask in its tensed condition.
Color cathode ray tubes have been in existence for many years. The so-called shadow mask type of color tube is most popular and is generally constructed with a target or screen consisting of a regular pattern of photo-deposited triads of red, green and blue light-emitting phosphors. The shadow mask is foraminous and positioned a predetermined distance from the target and, by virtue of its pattern of apertures, effectively shadows all but selected ones of the individual light-emitting phosphors from its corresponding electron beam-emitting source located in the neck of the tube. Since each tube's shadow mask is used in photoscreening the phosphor target therein, means for enabling repeatable, precise interregistration of the shadow mask and tube faceplate is a necessity. In most color cathode ray tubes, the shadow mask comprises a domed, foraminous metal sheet supported on a heavy frame. The mask is capable of self-maintaining its configuration and is repeatably positioned in registration with the tube panel by use of a plurality of studs embedded in the panel, and a corresponding plurality of stud-engaging flat springs attached to its supporting frame. The apparatus and screening techniques for such tubes are all well-known in the art and will not be described further herein.
The limitations of the shadow mask with respect to power-handling capability and image resolution are well-known. This invention concerns a novel "tensed mask" tube which utilizes a mask in the form of a thin foil having a pattern of suitable apertures. The mask is held under tension in a position adjacent to a flat or cylindrical phosphor screen-bearing face panel or faceplate. Since the foil mask is extremely thin, etching of the apertures therein can be very precisely controlled, thus making a higher resolution tube. Also, since the foil is under significant tension; that is, close to the deformation limit of the material used, it may be heated to a high temperature before the tension is relieved and mask distortion sets in. The electron energy that a tensed foil mask can absorb without significant degradation in color purity is much higher than for a conventional spherical shadow mask tube. Consequently, a much brighter, sharper image is possible with a tensed mask tube.
One of the inherent problems in a tensed mask tube is that the support structure for the mask must be capable of withstanding the relatively large tensile forces exerted by the mask thereon without substantial deformation. One mask-supporting technique described and claimed in the referent copending applications involves attaching the periphery of the mask to a glass support frame by means of a devitrifiable glaas, or "frit". The mask is typically composed of a steel having a coefficient of thermal expansion selected to be significantly higher than that of glass. The mask is sustained under modest tension and joined to the support frame with a bead of devitrifiable frit. The assembly is heated to a temperature at which the frit devitrifies. During the heat cycle, the steel mask expands at a much greater rate than the glass support frame, and when the frit devitrifies, it firmly bonds the periphery of the mask to the support frame. The mask is prevented from returning to its original size, and thereby is subjected to high tension forces; e.g., 30,000 psi.
As alluded to above, in accordance with conventional photographic techniques for fabricating the phosphor screens on the face panels of color tubes, the shadow mask is used as a stencil and must therefore be repeatedly positioned in accurate registration with the panel and with respect to the light source that simulate the electron beams developed by the electron gun. It will be appreciated that any deformation in the supporting structure for the tensed mask will result in corresponding distortions in the tensed mask aperture pattern during screening, and a corresponding distortion in the phosphor screen formed.
In accordance with the referent copending applications, the tensed mask support frame ultimately becomes a structural element of the tube envelope. During the assemb1y process, the entire tube structure, including panel, support frame and funnel, are subjected to an elevated temperature with devitrifiable frit on corresponding adjoining surfaces. When the frit devitrifies, a unified glass structure results which is hermetically sealed against the atmosphere. As the support frame and tensed mask are heated during the assembly process, the mask expands more than the frame, and the tension in the mask is greatly reduced. Deformation in the mask support frame is correspondingly reduced. As the devitrifiable frit vitrifies at the sealing temperature, the support frame is captured between the rigid panel and funnel or envelope portions of the tube. Upon subsequent cooling, the tension forces are again exerted by the tensed mask on the frame, but do not result in deformation of the support frame because it is now firmly supported by the face panel and the funnel. Thus the aperture pattern in the tensed mask of the finished tube differs from the distorted aperture pattern in the tensed mask when it was used to form the tube phosphor screen. Because of this fact, color impurity is apt to occur.
U.S. Pat. No. 3,894,321 to Moore, of common ownership herewith, is directed to a method for processing a color cathode ray tube having a thin foil mask sealed directly to the bulb. lncluded in this disclosure is a description of the sealing of a foil mask between the juncture of the skirt of the faceplate and the funnel. The foil mask is noted as having a greater thermal coefficient of expansion than the glass to which it is mounted, hence following a heating and cooling cycle in which the mask is cemented at the funnel-faceplate juncture, the greater shrinkage of the mask upon cooling places it under tension. The mask is shown as having two or more alignment holes near the corners of the mask which mate with alignment nipples in the faceplate. The nipples pass through the alignment holes to fit into recesses in the funnel. In another embodiment, the front panel is shown as having an inner ledge forming a continuous path around the tube, the top surface of which is a Q-distance away from the faceplate for receiving the foil mask such that the mask is sealed within the tube envelope. An embodiment is also shown in which the faceplate is skirtless and essentially flat.
The following patents are also noted. No. 1,163,495 (GB); U.S. Pat. Nos. 2,761,990; 3,440,469; 3,638,063; 3,873,874; 3,894,321; 4,069,567; and 4,495,437.